Under certain operating conditions, engines that have high compression ratios, or are boosted to increase specific output, may be prone to low speed pre-ignition combustion events. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Strategies have been developed for prediction and/or early detection of pre-ignition based on engine operating conditions. Additionally, following detection, various pre-ignition mitigating steps may be taken.
The inventors herein have recognized that a cylinder's propensity for pre-ignition may not only be affected by engine operating conditions, but also by a cylinder's pre-ignition history. For example, a cylinder that has already had a larger number of previous pre-ignition events may be more likely to pre-ignite than a cylinder that has had a fewer number of previous pre-ignition events. Consequently, pre-ignition mitigating steps that are effective for the cylinder with the fewer number of past pre-ignition events may not be as effective for the cylinder with the larger number of past pre-ignition events. In other words, a more aggressive approach to pre-ignition mitigation may be required for some cylinders as compared to other cylinders.
Thus, in one example, the issue may be addressed by a method of operating an engine comprising, in response to a first condition with a first number of pre-ignition events, adjusting operation of a first cylinder in response to an indication of pre-ignition in the first cylinder. Then, in response to a second condition with a second, higher, number of pre-ignition events, adjusting operation of a first and second cylinder in response to the indication of pre-ignition in the first cylinder.
In one example, an engine controller may store a pre-ignition history of each engine cylinder, including a number of pre-ignition events, in a pre-ignition database. The number of pre-ignition events may include, for example, a cylinder pre-ignition count, including a number of cylinder pre-ignition events that have occurred over the lifetime of the cylinder's operation (cylinder lifetime pre-ignition count), a number of cylinder pre-ignition events that have occurred over the current and/or immediately preceding engine cycle (cylinder trip pre-ignition count), a number of continuous pre-ignition events in the cylinder (consecutive cylinder pre-ignition count), a number of pre-ignition events that have occurred over the lifetime of the engine's operation (engine lifetime pre-ignition count), a number of pre-ignition events that have occurred over the current and/or immediately preceding engine cycle (engine trip pre-ignition count), etc. The pre-ignition history may also include details pertaining to mitigating steps previously taken in response to previous pre-ignition events, as well as their effectiveness at addressing the cylinder's pre-ignition. Based on the pre-ignition count of the cylinders, and further based on engine operating conditions, the controller may anticipate a likelihood of pre-ignition, and preemptively limit an engine load at the onset of engine operation. For example, the engine load may be limited more as a number of previous pre-ignition events increases.
However, even after preemptively limiting the engine load, cylinder pre-ignition may occur. Thus, in response to an occurrence of cylinder pre-ignition, the engine controller may update the pre-ignition history by increasing the pre-ignition count(s). Then, to immediately address the pre-ignition, a cylinder enrichment may be performed. Specifically, the enrichment may be based on the updated pre-ignition count. For example, the enrichment may include a degree of richness of cylinder operation, as well as a duration of operating rich, and as the pre-ignition count increases (or exceeds a threshold), a degree of richness and/or a duration of the enrichment may be increased. At the same time, the engine load may also be further limited based on the pre-ignition count. For example, engine load may be limited by reducing air flow to the cylinder, such as by reducing a throttle opening, reducing engine boost, and/or adjusting a cam timing. By enriching the cylinder in response to the occurrence of pre-ignition, an immediate cylinder air charge cooling effect may be achieved that may reduce the occurrence of further abnormal combustion events. The simultaneous limiting of engine load, for example by reducing air flow, can further assist in reducing the occurrence of additional pre-ignition events. However, the effect of load limiting on pre-ignition may be delayed until a stable air flow is reached.
In one example, the load limiting may be synchronized with the enrichment by performing the load limiting at a ramp-in rate that is coordinated with the enrichment operation. For example, the ramp-in rate may be adjusted such that ramping in of the limited load is completed concurrent to completion of the enrichment. In another example, the ramp-in rate may be adjusted based on the pre-ignition count.
Further still, in addition to the enrichment and load limiting, spark timing may be advanced by an amount. Specifically, spark may be advanced, relative to the spark timing at the time of pre-ignition detection, towards MBT. The amount of spark advance may be adjusted based on the current engine speed and/or the enrichment. Thus, as the degree of richness and/or duration of enrichment increases, the amount of spark advance may be increased. Since the cylinder may be more tolerant to spark advance due to the richer than stoichiometry air-to-fuel ratio during the enrichment, spark advance may be advantageously used in conjunction with the enrichment to maintain IMEP under the rich conditions of the cylinder.
Following the enrichment, the cylinder may be operated lean, the enleanment based on the preceding enrichment. For example, as a degree of richness and/or duration of the preceding enrichment increases, the degree of leanness and duration of the enleanment may be increased. As such, the preceding pre-ignition mitigating enrichment can lead to a drop in exhaust feedgas oxygen content, which can, in turn, degrade the catalytic efficiency of an emission control device catalytic converter, thereby degrading exhaust emissions. By enleaning the cylinder based on the enrichment, exhaust feed-gas oxygen levels may be returned within catalyst operational ranges, and improving exhaust emissions. Following the enleanment, the cylinder may resume stoichiometric operation.
The pre-ignition count may include, for example, a cylinder lifetime pre-ignition count (that is, a count of a total number of pre-ignition events over the life of each cylinder of the engine), a cylinder trip pre-ignition count (that is, a count of total number of pre-ignition events in each cylinder over the current engine cycle), an engine pre-ignition count (that is, a count of total number of pre-ignition events in the engine), as well as a consecutive pre-ignition count (that is, a count of a total number of consecutive and uninterrupted pre-ignition events over a plurality of consecutive combustion events). The enrichment and load limiting may also be adjusted differently based on the different pre-ignition counts. In one example, a more aggressive approach may be taken in response to the cylinder trip pre-ignition count exceeding a threshold (such as, by enriching more rich and/or for a longer duration, and by limiting more load) as compared to the cylinder lifetime pre-ignition count exceeding a threshold (such as, by enriching less rich and/or for a shorter duration, and by limiting less load). In another example, the load limiting and enrichment may be of a smaller amount when the cylinder pre-ignition count exceeds a threshold and by a larger amount when the engine pre-ignition count exceeds a threshold. In still another example, as the consecutive pre-ignition count exceeds a threshold, load limiting as well as a degree of richness and duration of the enrichment may be increased.
Further still, in response to pre-ignition in a given cylinder, the enrichment of other cylinders and engine load limiting may be adjusted based on the pre-ignition count of the given cylinder. For example, in response to a first condition with a first, lower number of previous pre-ignition events, the operation of a first cylinder may be adjusted responsive to an indication of pre-ignition in the first cylinder. In comparison, in response to a second condition with a second, higher number of previous pre-ignition events, the operation of a first cylinder and a second cylinder may be adjusted responsive to an indication of pre-ignition in the first cylinder.
In another example, the pre-igniting cylinder may be a cylinder in a first group (or bank) of cylinders, the engine further including a second group of cylinders. Thus, in response to a cylinder pre-ignition count (such as a cylinder consecutive pre-ignition count) of a first cylinder in the first group exceeding a threshold, a camshaft timing of the first group of cylinders may be adjusted to limit the engine load of the first group by a higher amount while the camshaft timing of the second group is maintained, or adjusted so as to limit the engine load by a smaller amount. Similarly, all cylinders of the first group, but not the second group, may be enriched. Alternatively, the cylinders of the first group may be enriched more than the cylinders of the second group. In yet another example, the enrichment and load limiting of a given group of cylinders may be adjusted based on the pre-ignition count of an affected cylinder of that group. That is, in response to pre-ignition in a first cylinder in the first group and a second cylinder in the second group, the enrichment and load limiting of the first group, but not the second group, may be adjusted based on the pre-ignition count of the first cylinder, while the enrichment and load limiting of the second group, but not the first group, may be adjusted based on the pre-ignition count of the second cylinder. Still other combinations may be possible.
As such, since the pre-ignition count correlates with a cylinder's propensity for further pre-ignition, by adjusting enrichment and load limiting profiles for the affected cylinder as well as the other cylinders of the engine based on the pre-ignition count, pre-ignition may be better anticipated and addressed.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.